Heating apparatus



Jan. 12, 1943.

R. A. BIERWIRTH HEATING APPARATUS Filed Nov. 29, 1941 g; hi9 baa/Aura 5591 t??? W101i.

l'crw Zhwentor torucg Patented Jan. 12, 1943 UNITED STATES PATENT OFFICE HEATING APPARATUS Rudolph A. Bierwirth, Haddon Heights, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application November 29, 1941, Serial No. 420,964

9 Claims.

place the wood between a pair of conducting plates or electrodes constituting a capacitor between which the wood laminations constitute a dielectric, and impressing a radio frequency voltage of suitable frequency on the plates. It has been found that, in order to heat the wood to the desired temperature in a reasonably short time, a frequency of 10 megacycles or more is required to prevent voltage breakdown. When long pieces of wood are glued together, the length of the plates becomes appreciable compared to the wave length at the operating frequency, and standing waves are set up which result in uneven heating.

The primary object of my present invention is to provide improved heating apparatus of this type which will not be subject to the aforementioned disadvantage.

More particularly, it is an object of my present invention to provide an improved wood-gluing machine in which the wood will be heated uni formly throughout its length and even gluing obtained.

Heating apparatus of this type is generally supplied with energy from a radio frequency source through a concentric line the impedance of which is relatively high. On the other hand, the impedance of the electrodes is relatively low and does not match the impedance of the input line, so that a relatively large current is ordinarily required. It is another object of my present invention, therefore, to provide an improved heating apparatus of the type under considera tion wherein the impedance of the load has a value such that it can be coupled efficiently through a conventional line to the radio frequency source, so that the current requirement is reduced.

A further object of my present invention is to provide improved heating apparatus as aforesaid which is simple in construction and highly efficient in use.

In accordance with my invention, I connect framework of the press.

associated capacitor section a parallel tuned network which is tuned to resonance at the operating frequency. These networks have the effect of confining the deviation in voltage along the line consisting of the two, long electrodes from the maximum voltage applied thereto by the standing waves to a small fraction (for example,

' 5 per cent) of this maximum whereby to substantially uniformly distribute the voltage throughout this line, thereby overcoming the eflect of the standing waves and providing substantially uniform dielectric losses through the wood structure along the length thereof. In this way, uniform heating of the wood is assured. Also, these networks raise the impedance of the load to substantially the impedance of the input line, so that the current requirement is greatly reduced. In general, the number of inductors employed will be a function of the operating frequency of the radio frequency source and the length of the electrodes, and the deviation from maximum voltage should preferably not substantially exceed 5 per cent.

The type of press employed in connection with my invention includes a bed plate on which one of the electrodes rests and a head which is ad- -justab1e relative to the bed plate. The wood or other material to be glued rests on the firstnamed electrode and the second or cooperating electrode rests on top of the laminated wood structure. Between the top electrode and the head of the press, a long strip or block of insulating material is placed, the head of the press being coupled to the bed plate thereof through the In accordance with a further feature of my invention, I connect a plurality of additional inductive tuning elements between the upper electrode and the head of the press, all suitably spaced along the length of the upper electrode and the press head, these elements being adjusted to tune the capacity between the upper electrode and the press head to parallel resonance at the operating frequency.

, This reduces the current circulating through the press framework and maintains a more uniform voltage distribution on the upper electrode, thereby increasing the efficiency of the apparatus.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description of several embodivention, Figure 2 is a top plan view of the electrode and I network structure,

Figure 3 is an enlarged sectional view taken on the line IIIIII of Figure 1,

Figure 4 is a wiring diagram of the circuit employed,

Figure 5 is a fragmentary view of a press formed in accordance with a modified form of my invention, and

Figure 6 is a plan view similar to Figure 2 of still another form of my invention.

Referring more particularly to the drawing, wherein similar reference characters designate corresponding parts throughout, there is shown a press comprising a grounded bed plate I carrying a framework 3 which supports the head 5 of the press through a plurality of rams or the like I by means of which the head 5 is vertically adjustable relative to the bed plate I. Resting on the bed plate I is an elongated, lower plate electrode 9 of copper or any other suitable material on which may be placed the several layers or laminations of wood or any other dielectric material II to be bonded together by a thermosetting glue or the like I3. A second, elongated plate electrode I5 is placed on top of the wood strips II, and a long block of wood or other suitable dielectric material I1 is interposed between the electrode I5 and the press head 5.

When the head of the press is brought down on the block I1 and the pressure is applied to the assembly through the rams I, the laminations II and the intervening glue [3 are brought into firm engagement with each other.

Radio frequency energy from a suitable oscillator I9 may be supplied to the electrodes 9 and I5 through a suitable input line, such as a concentric cable 2I, the outer conductor of which is connected to ground through the lower electrode 9 and the bed plate I, and the inner conductor of which is connected to the upper electrode I5. In the system thus far described, it is found that standing waves are set up along the length of the electrodes and the laminated wood structure I I. This results in uneven heating and uneven gluing of the laminated wood structure. To avoid this, I connect across the electrodes 9 and I5 a plurality of inductors 23 in shunt relation, as clearly shown in Figure 4. The inductors 23 may comprise a pair of copper tubes 25 and 21 connected, respectively, to the electrodes 9 and I5 along the front edges there-.

of, and bridged by an adjustable, conductive connecting link 29. Each of the inductors 23 is associated with a predetermined section of the electrodes 9 and I5 and forms, therewith, a parallel resonant network which may be tuned to resonance at the operating frequency by adjusting the link 29 along the tubes 25 and 21. These parallel networks have the efiect of substantially uniformly distributing the voltage along'the lengths of the electrodes whereby an even distribution of dielectric losses through the laminated wood structure along the length thereof is obtained, thus insuring uniform heating and uniform gluing of this structure.

In one press constructed according to the present invention, the electrodes 9 and I5 were 13 feet long and 6 inches wide, and six tuning elements 23 were provided, the end elements 23 being spaced 13 inches from the respective ends of the electrodes 9 and I5, and the other elements 23 being spaced 26 inches apart from each other. The electrodes were thus divided into six equal sections with each of which an inductor 23 was associated. The oscillator was operated at a frequency of 45 megacycles. Each tuning element 23 had suflicient inductance to tune ,-6 of the total capacitance of the plates to parallel resonance at this frequency. The wood load II consisted of 3 spruce boards 1% inch thick, 6 inches wide, and 13 feet long, so that the total thickness of wood was {-3 inch. This load between the electrodes 9 and I5 had a capacitance of about 700 mtf. which has a reactance of about 5 ohms at 45 megacycles and a Q of about 30. Each tuning element 23 tuned with /2; of 700 wi or approximately 117 t, which required an inductance of 0.106 H. Under these conditions, a variation of voltage of less than 6 per cent was observed.

The tuning elements 23 not only insure uniform heating of the wood load II, but raise the impedance of the load to a value which matches the relatively higher impedance of the input line 2|. To prevent a large current from flowing through the head 5 and the frame 3, there may preferably also be provided a number of adjustable tuning inductors 30 similar to the tuning elements 23 and each comprising a copper tube 3I connected to the upper electrode I5, a copper tube 33 connected to the press head 5, and an adjustable conductive link 35 bridging the copper tubes 3I and 33, as clearly shown in Figure 3. The tuning elements 30 are adjusted to parallel resonance with the capacity between the electrode I5 and the press head 5, thereby increasing the impedance between the electrode I5 and the head 5 and reducing the current circulating through the framework 3. A further advantage obtained in using the tuning inductors 30 is that a more uniform voltage distribution is maintained on the upper electrode I5. Any suitable number of tuning inductors 30 may be employed, the press above referred to having been supplied with four such inductors of which the end ones were spaced 19 inches from the ends of the electrodes 9 and I5, and the intermediate ones of which were spaced about 39 inches apart. Thus, each element 30 tuned /4 of the total length.

The arrangement shown in Figures 1 and 3. where in the inductors 23 are disposed along the' front edges of the electrodes 9 and I5, necessitates inserting and subsequently withdrawing the Wood laminations I I through the back of the press. To provide an arrangement by which a continuously flowing operation may be obtained, that is, where the wood load may be inserted from the front, for example, and subsequently withdrawn from the back, the modification shown in Figure 5 may be employed. In this modification, only two tuning inductors 31 are employed, each consisting of a copper tube 39 secured at one end to the lower electrode 9 and having its opposite end connected to the upper electrode I5 by an adjustable link 4| which may be adjusted longitudinally along the respective tubes 39 and the electrode I5. In this modification, each of the inductor elements 37 tunes /2 of the total length of the electrodes 9 and I5.

The modifications thus far described are all suitable for relatively narrow strips of wood. Where wide strips are to be glued, it will be necessary, of course, to employ correspondingly wide electrodes, and in such case, the tuning elements 23 may be placed along both longitudinal edges of the electrodes 9 and I5, as shown in Figure 6.

Although I have shown and described several embodiments of my invention, it will undoubtedly be obvious to those skilled in the art that many other modifications thereof, as well as changes in those described, are possible. For example, if desired, the bed plate I of the press may be used as the lower electrode and the electrode 9 thus omitted. Other similar changes may also be made.

I claim as my invention:

1. In heating apparatus employing high frequency electrical energy, the combination of a pair of elongated electrodes spaced from each other to constitute a capacitor, means connected to said electrodes and providing an external connection to a source of said energy, said electrodes having a length which is appreciable compared to the wave length of said energy, and a plurality of tuning inductors connected in shunt relation across said electrodes at predetermined points therealong.

2. In heating apparatus employing high frequency electrical energy, the combination of a pair of elongated electrodes spaced from each other to constitute a capacitor, means connected to said electrodes and providing an external conneetion to a source of said energy, said electrodes having a length which is appreciable compared to the wave length of said energy, and a plurality of tuning inductors connected in shunt relation across said electrodes at spaced points therealong, the spacing of said points being proportional to the frequency of said energy.

v3. In heating apparatus employing high fre quency electrical energy, the combination of a pair of elongated electrodes spaced from each other to constitute a capacitor, means connected to said electrodes and providing an external connection to a source of said energy, said electrodes having a length which is appreciable compared to the wave length of said energy and being nor-' mally subject to standing waves when supplied with said energy, and reactance means associated with said electrodes at spaced points therealong for so altering the impedance between said electrodes at said points that the variation in voltage along the length of said electrodes will be a small fraction of the maximum voltage of said standing waves.

4. The invention set forth in claim 3 characterized in that said fraction is not substantially in excess of 5 per cent.

5. The invention set forth in claim 3 characterized in that said last-named means comprises a plurality of inductors connected in shunt relation across said conductors at said spaced points whereby each of said inductors cooperates with a predetermined section of each of said electrodes to provide a parallel tuned network at substantially the frequency of said energy.

6. The invention set forth in claim 3 characterized in that said last-named means comprises a plurality of variable inductors connected in shunt relation across said conductors at said spaced points whereby each of said inductors cooperates with a predetermined section of each of said electrodes to provide a parallel tunable network, said networks being tunable to alter the impedances between said electrodes at said points to thereby alter the voltages across said electrodes at said points. v

7. In apparatus for gluing together by means of high frequency electrical energy a laminated structure consisting of elongated strips of dielectric materials, the combination of a press including a bed plate and a head adjustable relative to said bed plate, a first electrode supported on said bed plate, a second electrode in spaced relation to said first electrode, said strips being disposed between and in engagement with said electrodes and said electrodes having a length substantially equal to that of said strips, a member of insulating material disposed between said second electrode and said head, means connected to said electrodes and providing an external connection to a source of said energy, said electrodes having a length which is appreciable compared to the wave length of said energy and being normally subject to standing waves when supplied with said energy, and a plurality of inductors connected across said electrodes in shunt relation at predetermined spaced points therealong whereby each of said inductors cooperates with predetermined sections of each of said electrodes to provide parallel tuned circuits at substantially the frequency of said energy to thereby provide substantially uniform dielectric losses through said strips along the length thereof and thus insure uniform heating of said strips throughout their lengths.

8. The invention set forth in claim 7 characterized by the addition of a plurality of additional inductors connected in shunt relation across said second electrode and said head, each of said lastnamed inductors being tuned to parallel resonance with predetermined sections of each of said constitute networks which provide a relatively high impedance to said external connection means to thereby reduce the current requirement.

RUDOLPH A. BIERWIRTH. 

